The present invention relates to a single crystal pulling apparatus for growing and pulling up a single crystal based on the Czochralski method, and more particularly the invention relates to an improved single crystal pulling apparatus of the kind wherein the heater is adapted to shift vertically.
A single crystal pulling apparatus which adopts the CZ (Czochralski) method consists mainly of a main chamber wherein the single crystal is grown and a pull chamber. In the main chamber are provided such elements as a crucible for containing the polycrystal substance (a raw material such as silicon) to be single-crystallized, a cylindrical heater surrounding the crucible, and a cylindrical thermal insulator (heat shield) surrounding the heater. Beneath the main chamber are provided mechanisms for rotating the crucible about a vertical crucible shaft, on which the crucible is fixedly mounted, to control the uniformity of the heat flux in the molten polycrystal substance during the crystal growing operation. Installed above the main chamber is a towering structure of the pull chamber which is an upright, substantially cylindrical enclosure. An isolation valve is provided between the main chamber and the pull chamber. The single crystal ingot which is grown and raised high in the main chamber is brought up into the pull chamber.
The polycrystal substance charged in the crucible is melted down by the heater to turn into a polycrystal molten liquid, and in this liquid is dipped a seed crystal fixed at the lower end of a pull means such as a wire, and the desired single crystal grows from the seed crystal as the pull means is rotated and drawn up at predetermined rates together with the seed crystal.
In such a conventional single crystal pulling apparatus, if the crucible were held and kept at the original altitude, the surface level of the molten liquid in the crucible would gradually shift downwards relative to the heater as well as the crucible wall with the growth of the single crystal, which results in an undesirable change in the heating condition to which the solid/liquid interface between the grown crystal and the molten liquid is subjected.
Therefore, with the view to maintaining the altitude of the surface level of the molten crystal the crucible shaft is adapted to vertically shift and thereby displace the crucible such that the downward displacement of the melt level (solid-liquid interface) is always compensated for by the measured rise of the crucible; in particular, the crucible is raised at a predetermined rate by means of a drive means.
However, in the conventional single crystal pulling apparatus, the heater is stationary so that when the liquid level shifts downwards relative to the crucible wall and the crucible is lifted, the positional relationship between the heater, the crucible wall and the solid-liquid interface changes, whereby the thermal effects of the heater on the solid-liquid interface changes and this typically results in lowering of the oxygen concentration in the growing single crystal. Thus, in a single crystal ingot grown in a conventional single crystal pulling apparatus, the oxygen concentration distribution in the ingot tends to be such that the closer to the bottom of the ingot the lower the oxygen concentration.
In order to solve this problem, an improved single crystal pulling apparatus was proposed wherein the heater is adapted to shift vertically and this vertical shifting of the heater is controlled to correspond the rise of the crucible.